A former official from several Republican administrations said he believes President TrumpDonald John TrumpSunday shows preview: Trump sells U.N. reorganizing and Kavanaugh allegations dominate Ex-Trump staffer out at CNN amid “false and defamatory accusations” Democrats opposed to Pelosi lack challenger to topple her MORE’s repeated attacks on special counsel Robert MuellerRobert Swan MuellerSasse: US should applaud choice of Mueller to lead Russia probe MORE and the Russia investigation are a sign that Trump is worried the investigation is closing in on him.

Peter Wehner, who worked in President Reagan’s administration and both Bush administrations, said Monday on CNN that the stronger the case is that Mueller has, “the more vitriolic, the more acrimonious, the more libelous” Trump and his supporters will be in their attacks on the special counsel.

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“It’s actually a sign of the strength of the case that Mueller is assessing and their fear of what he’s going to reveal,” Wehner said. “If Robert Mueller didn’t have anything, they would ignore him. But they know that he’s got something. And they can’t refute it on the facts, so they have to try and delegitimize him and the investigation.”

Wehner added that he doesn’t think Trump’s attacks on Mueller will prove successful.

“In the end, Robert Mueller is going to come forward with his report,” he said. “My suspicion is that it’s going to be a very powerful indictment that’s going to demonstrate layers and layers of wrongdoing.”

Trump has frequently lashed out at Mueller and his investigation into Russian interference in the 2016 election and possible collusion between Russia and the Trump campaign.

Trump has called the investigation a “witch hunt” and earlier this month called on Attorney General Jeff Sessions to “stop this Rigged Witch Hunt right now.”